Method and plant producing a bituminous paving mixture

ABSTRACT

A drying and mixing plant for producing a bituminous paving mixture comprises a drying and mixing drum (10) which is supported by a hydraulic cylinder (16) or other lifting means so that the inclination of the drum axis may be altered. By choosing a suitably small angle of inclination so that the drum inclines downwardly towards its outlet end, the plant may be used for continuous operation, so-called drum-mixing. Aggregates and other components may then be supplied continuously to the inlet end of the drum from silos (18), through a center intake (24) from another source (25) and binder such as bitumen may be supplied through a supply tube (58). Finished paving mixture is continuously discharged from the outlet end of the drum and passed to a product silo (28) via a conveyor (27). Alternatively, if the drum axis is arranged substantially horizontally, it is possible to treat two or more substantially separate material portions of different compositions in the drum (10) at the same time in so-called batch-mixing. By tilting the drum (10) by means of the cylinders (16), the material portions may be transferred from one drum section to the next after a certain time of treatment, and finished paving mixture may be discharged as a total portion from the drum section adjacent to the outlet end of the drum. The discharged mixture portion may possibly be further treated or be mixed with further materials in a mixing apparatus (30), to which the mixture portion is passed by means of a conveyor (29). The same plant may thus optionally be used for continuous production of large amounts of a paving mixture of the same composition or for batch-wise production of small asphalt amounts of the different compositions.

The present invention relates to a method of producing a bituminous orasphalt paving mixture.

So far, practically all bituminous paving mixture has been produced byeither one of two different methods, namely the so-called "batch-mixing"and the so-called "drum-mixing", respectively. Batch-mixing is adiscontinuous method of production which is suitable for use inconnection with production plants from which relatively small amounts ofa bituminous or asphalt mixture of varying, further specifiedcompositions are to be delivered during a day to many differentpurchasers. In the known batch-mixing, stone and sand fractions aremetered according to the purchaser's recipe or specifications by meansof metering apparatus, and the materials so metered are transported to adrying drum where the materials are dried and heated to a temperature ofabout 160°-180° C. Then the materials are sieved into 4-6 fractions andstored in a hot silo or buffer silo. From the hot silos a portion orbatch of 2000-4000 kg is weighed out, and filler, bitumen and otheradditives are weighed out as a certain percentage of the stone and sandmaterial according to the recipe presented. When all components havebeen weighed out, the materials are mixed for about 30-50 seconds in aspecial mixing apparatus. The mixture is now ready for use and may betransported to a product silo where it may be collected by thepurchaser.

Drum-mixing is a continuous method of production which is suitable foruse in connection with production plants from which large amounts ofbituminous or asphalt paving mixtures of rather uniform compositions areto be supplied, for example in connection with large civil engineeringprojects or for purchasers of large asphalt mixture amounts. Indrum-mixing, aggregate, such as stone and sand fractions are meteredcontinuously in accordance with a recipe or specifications by means ofmetering apparatus, and the metered materials are continuously passed toa drying drum through which they may flow substantially continuously. Inthe first part of this drum the materials are dried and heated to atemperature of 160°-180° , and in the last part of the drum, filler,bitumen and any additives are continuously added. The dried and heatedmaterials are mixed with the additives in the last part of the drum sothat finished mixture is discharged continuously from the outlet end ofthe drum.

It will be understood that the known production plants for pavingmixtures are rather inflexible, as a certain plant only permits eitherproduction of relatively small amounts of mixtures of variouscompositions or continuous production of a large amount of a pavingmixture of the same composition. Therefore, a paving mixturemanufacturer who wants to be able to produce large as well as smalleramounts of paving mixtures of a desired compositions has to invest in atleast two different plants for batch-mixing and drum-mixing,respectively.

The invention provides a method which renders it possible to producelarge amounts as well as relatively small amounts of paving mixture ofdifferent compositions in a rational manner in the same plant.

The method according to the invention comprises supplying aggregate, forexample sand and/or gravel, into a first section of a rotating dryingand mixing drum at an inlet end thereof, introducing at least onefurther component, for example using paving material, a binder, such asbitumen and/or recycled material into a second section of the drumbetween said inlet end and an opposite outlet end of the drum, mixingsaid component with the dried aggregate in the drum, and discharging themixture at the other end of the drum, and the method according to theinvention is characterized in supplying the aggregate to the drum inportions, drying each portion in said first drum section, transferring afirst portion to the second drum section after drying such portion andprior to supplying a second portion of aggregate into the drum, andmixing the dried first portion of aggregate with said further componentin said second drum section, while said first and second portions arekept substantially apart in the drum.

When two or more portions of aggregate are thus heated simultaneously inone and the same drum and kept substantially separated therein, and thefinished mixture is likewise discharged in portions from the outlet endof the drum, this is a case of batchmixing. However, if aggregate andthe further component, such as binder and/or other additives, aresupplied continuously to the same drum it may be operated as inconventional drum-mixing. The method according to the invention thusrenders possible the use of one and the same plant according to bothsaid known mixing principles.

It is possible to treat more than two aggregate portions in the drum atthe same time. Thus, a plurality of separate aggregate portions may betreated in various sections of the drum.

These drum sections are normally longitudinal sections of one and thesame drum, but, alternatively, the drum may be formed as a drum unitcomposed by separate drum sections connected in series.

The dried aggregate portions may be transferred from one section to theother in any suitable manner. The aggregate portions may, for example,be pushed from one section to the other by means of some kind ofautomatically operating pushing means arranged inside the drum. However,the transfer of the dried aggregate portions from one section to anothersection and possibly also discharging the bituminous mixture from thedrum is preferably effected by tilting the drum and/or by increasing therotational speed of the drum. The tilting may, for example, be effectedby supporting one end of the drum by lifting means, for examplemechanical jacks or hydraulic or pneumatic cylinders which are adaptedto selectively lift or lower one end of the drum.

During the drying and mixing process in the drum the drum axis may behorizontal or may define a small acute angle with a horizontal plane inan upward or downward direction, and when a dried aggregate portion isto be displaced from one drum section to another, this may be effectedby lifting one end of the drum so that the aggregate portion may slideforward to the next drum section, while a finished mixture portion maypossibly simultaneously be discharged from the drum. Between adjacentdrum sections, barriers may be arranged to ensure that the portionsbeing treated simultaneously in the drum are kept apart. These barriersmay, for example, be in the form of projections or ribs formed on theinner wall of the drum. These projections may, for example, form anannular, radially inwardly extending partition wall which may be openedwhen a portion is to be transferred from one drum section to another.Such a partition wall may, for example, be formed by a number of ringsectors which are mounted in the drum wall in such a manner that theymay be rotated about substantially radial axes. When a material portionis to be transferred, these rings sectors may be rotated from a positionin which they are at substantially right angles to the drum axis into aposition in which they extend substantially parallelly therewith.Furthermore, an end wall may be arranged at the outlet end of the drum.Such an end wall may be moved into an open position in case ofdrum-mixing, or when a portion of finished mixture is to be dischargedfrom the drum in case of batch-mixing.

The invention also relates to a drying and mixing plant for theproduction of a bituminous paving mixture, said plant comprising adrying and mixing drum having inlet and outlet ends, driving means forrotating the drum about its longitudinal axis, means for feedingaggregate into the drum at its inlet end, and means for supplying afurther component, such as a binder, into the drum at locationsintermediate of the inlet and outlet ends, and the plant according tothe invention is characterized in inclination adjustment means forlifting and lowering the drum so as to change the inclination of thedrum axis, and in closure means for selectively at least partly closingand opening the outlet end of the drum. Such a drum which is suitablefor drum-mixing may then also be used for batch-mixing, as the aggregatemay be introduced into the drum in portions which may then betransferred from one drum section to another or be poured out of thedrum by tilting the drum by means of the inclination adjustment means,when the closure means are open. The inclination adjustment means may,for example, be in the form of mechanical jacks or pneumatic orhydraulic cylinders. The transfer and the pouring out of the portions ofmaterial may be speeded up by increasing the rotational speed of thedrum and thus the material transportation velocity.

The end closure means may, for example, comprise an end wall which isoperated by means of hydraulic or pneumatic cylinders, screw spindles orother mechanical moving means. Such end wall may be movable between aclosed position in which the end wall is closely adjacent to and atleast partly closes the outlet end of the drum, and an open position inwhich the end wall is spaced from the outlet end so that the latter isopen. In batch-mixing the outlet end of the drum is normally kept closedby means of the movable end wall and is opened only when a portion offinished mixture is to be discharged from the drum. In case ofdrum-mixing, however, the end wall is constantly kept in its openposition. However, if the closure means or end wall in its closedposition closes the drum end only partly, the closure means may remainclosed even in case of drum-mixing.

The aggregate introduced into the drying and mixing drum may, inprinciple, be dried by supply of heat from heating means of any type orfrom any suitable heat source. However, in a preferred embodiment, theseheating means comprise at least one burner directed into the drum. Ifthe burner is adjusted so that an optimum heat economy is obtained indrum-mixing and at a certain continuous flow of material, it will not bepossible to obtain such an optimum heat economy for all sizes ofportions of raw material supplied to the drum when the same drum is usedfor a batch-mixing process. According to the invention the burner may bemounted in such a manner that its direction may be altered, for examplein the horizontal as well as in the vertical direction, and thedirection of the burner may then be adjusted so that the best possibleheat exploitation is obtained at each individual portion size. Theposition of the burner inside the drum may also optionally be changed,and the burner capacity may likewise be adjusted as desired.

The drum may be divided into sections by at least one annular partitionwall extending radially inwardly from the inner wall of the drum, saidpartition wall being selectively movable between a position in whichflow of material between adjacent drum sections is obstructed and aposition in which material may flow freely from one section to another.When the drum is used for batch-mixing, the partition wall is normallyin its closed position and then serves to divide the drum into sectionsand to keep apart the individual material portions being simultaneouslytreated in the drum. However, the partition wall may be opened when thematerial portions are to be transferred from one drum section to thesubsequent one. When the drum is used for drum-mixing, the partitionwall may be kept constantly open. The annular partition wall may bedivided into ring sectors of which at least some are movable between anobstructing position in which they extend transversely to the drum axis,and a non-obstructing position in which they extend substantiallyparallelly with or define an acute angle with the drum axis. These ringsectors may, for example, be swingable about radial axes from a positionin which they are at substantially right angles to the axis of the drum,into a position in which they extend substantially parallelly with orform an acute angle with the drum axis.

On the drum at one or more locations between its ends there may be oneor more material intakes through which material may be introduced in amanner known per se into drum sections positioned between the ends ofthe drum. Through such intakes, which may, for example, be in the formof material supply tubes or pipes which extend into the drum through oneof the ends of the drum in the longitudinal direction thereof and endinside the drum, materials, such as bitumen or another binder, additivesand the like, may be introduced into the drum. Through other intakes,recycled material may be supplied, for example in the form of oldcrushed asphalt pavement, and/or lime and so-called filler, which may,among other things, serve to increase the binding between bitumen andthe sand or stone materials, may be introduced. The means for feeding insuch further components may in a known manner comprise a peripherallyextending baffle wall projecting radially inward from the inner wall ofthe drum and defining an introduction chamber opening towards the outletend of the drum. Such an annular introduction chamber will cause acertain amount of material to be retained in the drum upstream of thisannular chamber. In drum-mixing this makes no great difference, as largeamounts of material of the same composition are treated continuously.However, in batch-mixing, such a retained amount of material from apreviously treated material portion will be able to "contaminate" a newmaterial portion of another composition. In order to reduce the amountof material retained by the baffle wall or the introduction chamber, thebaffle wall may be divided into sectors so as to define peripheral gapsbetween adjacent sectors. These baffle wall sectors preferably extend ina direction defining an acute angle with the longitudinal direction ofthe drum, whereby the baffle wall sectors may impart a movement to theintroduced material in a longitudinal direction towards the outlet endof the drum.

In drum-mixing the bituminous paving mixture is normally finished in themixing drum, and the finished material leaving the drum may therefore bepassed directly to, for example, a silo for finished product. When thesame drum is used for batch-mixing, it may be desired to subject themixture portions leaving the mixing drum to a further mixing process.Therefore, the outlet end of the drum may be connected to a mixingapparatus by means of a conveyor. This conveyor which may, for example,be a bucket conveyor, may be provided with weighing cells so that theamount of material supplied to the mixing apparatus may be accuratelydetermined. In addition to the dried and mixed materials arriving fromthe mixing drum, the mixing apparatus may, for example, be supplied withbinders, such as bitumen, filler, chemical additives, colouring agents,etc. Also the mass of these substances is determined accurately so thatthe desired composition of the finished product may be obtained. Thematerials to be mixed together, should preferably not stay in the mixingapparatus longer than at all necessary. On the other hand it is ofimportance that the mixed material becomes homogeneous. The problem maybe solved by incorporating measuring probes adapted to determine thepercentage content of one of the components at various locations in themixing chamber of the mixing appartus. When all the measuring probesregister substantially the same content of the component in question,the mixing process may be interrupted.

The mixing time may be determined empirically from a given amount ofmaterial and on the basis of a certain composition of materials. Whenthe mixing process has been finished, the finished mixture may bedischarged directly into a silo for finished product or into a buffersilo from which the material may be passed to a silo for finishedproduct or collected directly by the consumer. Even though it ispossible in batch-mixing to treat two or more material portions indifferent drum sections simultaneously, it is, of course, also possibleonly to treat a single material portion at a time in the drum.

The plant according to the invention preferably comprises an electroniccontrol device, for example in the form of a microcomputer, forcontrolling one or more of the following: The inclination of the drumaxis, the rotational speed of the drum, the rotational direction of thedrum, the position of the end closure means, the position of thepartition wall, the adjustment of the burner, the direction of theburner, the composition and temperature of the paving mixture produced,opening and closing of the end closure means and of the partition wall.

The memory of the computer may, for example, contain a large number ofrecipes for paving mixtures or asphalt compositions. The computer may beadapted to choose the optimum production method (drum-mixing orbatch-mixing), when the asphalt recipe and the amount of asphalt to beproduced have been entered. The computer may then be adapted to adjustvarious process parameters, including the angle of inclination of thedrum axis, the rotational speed and the direction of revolution of thedrying drum, the direction and adjustment in general of the burner, thesupply of bitumen, filler, additives, etc., and the temperature of theflue gas. The computer may further be adapted to automatically controlthe various silos containing the aggregate or raw materials used, suchas gravel and sand, and recycling materials, if any. Also the weighingout or metering of these materials may be carried out by the computer,including automatic taring of scales with an alarm for exceeding tarelimits, automatic afterflow correction for each metering gate ormetering valve, an alarm for exceeding the afterflow limit, monitoringthe correct functioning of all end stops, valves and gates, etc., and analarm in case of malfunction.

The flow speed of the various components which are added to the drum isnot the same. However, in order to ensure that all parts of the mixturebeing produced have the desired composition it is important that all ofthe components are added with such a timing that they all arrive at thefinal mixing zone of the drum at the same time. Therefore, the computeris preferably adapted to control the addition of the various componentsand the various fractions of the aggregate that all of the fractions andcomponents arrive at the final mixing zone at the same time, whereby itmay be obtained that also the first and the last part of a large mixtureportion produced by drum mixing is of the right composition. Such timingmay be made on the basis of empirical values. Because the flow rate ofaggregate may depend on the moisture condition thereof, the computer mayalso control the heating efficiency of the burner so as to obtain asubstantially uniform moisture content of the aggregate.

The control of the temperature of the finished mixture may be based onmeasurements of the temperature of the combustion gas exhausted from thedrum. The burner may be adjusted by adjusting the direction of theburner, controlling the amount of fuel supplied to the burner, and bycontrolling the surplus of air supplied to the burner based on ameasurement of O₂ or CO in the combustion gas.

The invention will now be explained more in detail with reference to thedrawings, in which

FIG. 1 shows a side view of an embodiment of a production plantaccording to the invention,

FIG. 2 shows the same as FIG. 1, where the inlet end of the mixing anddrying drum has been elevated,

FIG. 3 is a top plan view of the plant shown in FIGS. 1 and 2,

FIG. 4 is a diagrammatic, partially sectional side view of a part of theplant,

FIGS. 5 and 6 are cross-sections of various parts of the mixing anddrying drum of the plant, and

FIG. 7 diagrammatically shows part of a longitudinal section of themixing and drying drum in a material or component intake.

The production plant shown in the drawings comprises a drying and mixingdrum 10, which is so mounted that it may be rotated about itslongitudinal axis by means of driving motors 11, which, possibly via agear, drive a friction roll engaging with friction rings 12 arranged onthe outer peripheral surface of the drum. The drum 10 is rotatablymounted on a supporting frame 13 having one end connected to astationary support 15 via a hinge connection 14. The other end of thesupporting frame 13 may be lifted or lowered by means of one or morehydraulic cylinders 16 or mechanical lifting means, the frame 13 thenbeing rotated about its hinge connection 14, so that the direction orinclination of the longitudinal axis of the drum 10 may be changed. Aburner 17 is arranged at the inlet end of the drum 10. The burner 17 isdirected into the drum and is so mounted in relation to the drum thatthe direction of the burner may be changed in the horizontal as well asin the vertical direction in relation to the drum axis by means ofsuitable moving means, not shown. Further, the fuel flow fed to theburner and the ratio between the amounts of fuel and combustion air maybe adjusted to adjust the heating effect.

A number of aggregate or raw material silos 18 contains a number offractions, respectively, of sand and gravel material of well-definedgrain sizes. According to a given recipe, certain amounts of material oraggregate may be metered from the individual silos by means of meteringdevices 19, which may, for example, comprise discharge bands, the speedof which may be controlled, weighing cells, etc. These amounts ofmaterial or aggregate fall onto an underlying conveyor belt 20 fromwhich they are passed to a longitudinally extending conveyor 21extending from the outlet end of the conveyor belt to a further conveyor23 arranged on a higher level. The sand and gravel material, thecomposition of which with regard to grain size and other propertiescorresponds to the recipe, is now passed into the inlet end of thedrying and mixing drum 10 by means of the conveyor 23. Between its endsthe drum 10 comprises a material intake 24, by means of which one ormore further materials or aggregate, for example recycled material inthe form of old, broken asphalt pavement, filler, chemical additives,etc., may be introduced at a desired location of the drum. Two or moresuch material intakes may be provided in the drum, if desired. Thematerial(s) supplied to the intake 24 may be metered from a silo 25 andfrom the latter be passed to the intake 24 by means of a conveyor 26,vide FIG. 3. The design of the intake 24 will be described in furtherdetail below with reference to FIG. 7. Binders, such as bitumen and/orother liquid additives and any solid additives, such as fluxing agents,filler (filter dust, lime, etc.) and other additives which are mixedinto the liquid additives, may be introduced into the drum 10 via asupply tube or pipe 58 (vide FIG. 4). One end of the pipe 58 isconnected to a binder source, for example via a metering pump, notshown, and the other end of the pipe extends through one of the endopenings of the drum 10 into and along the drum so that the supply pipe58 opens into a section of the drum 10 where it is desired to introducethe binders and/or the additives. It is understood that solid additivesmay be blown into or pushed into the drum 10 through a similar supplypipe. Thus, for example filler material may be supplied into the drum 10from a filler silo 22 through one of the supply tubes 58. The supplypipe(s) 58 may be arranged so as to be longitudinally displaceable inrelation to the drum so that the position in the drum where theadditives are discharged, may be changed, for example depending onwhether the drum is used for drum-mixing or batch-mixing. The mixedproduct or the mixed asphalt leaving the outlet end of the drum 10 mayoptionally be passed either to a conveyor 27 or to a conveyor 29. Theconveyor 27 which may, for example, be a drag chain conveyor, passes themixture to a silo 28 for finished product, while the conveyor 29, whichmay, for example be a bucket conveyor, passes the mixture to a mixingapparatus 30.

The outlet end of the drum 10 opens into a chamber 31 the top of whichis connected to an air suction duct 32 communicating with a filteraggregate 33 which serves to filter off dust and other small solidparticles from the air and the combustion gases flowing through the drum10. An air duct 34 connects the filter aggregate 33 to a suction fan 35which is driven by an electric motor 36, and whose air outlet 37 isconnected to a chimney 38. As will be explained in detail below, theoperation of the plant and the various functions of the individualcomponents may to a large extent be controlled by an electronic controldevice, such as a microcomputer 39.

As it appears from FIG. 4, a buffer container or silo 41, which may beprovided with weighing cells, may be arranged above the mixing apparatus30 provided with mixing rotors 40. The inlet 42 of a conveyor 43 whichmay lead to finished product silos, not shown, is arranged immediatelybelow the mixing apparatus 30. As shown in FIG. 4, the conveyor 29, thebuffer container 41, the mixing apparatus 30, the conveyor inlet 42 andthe outlet end of the drum 10 may be arranged in a closed housing 44which may possibly replace the chamber 31. In such a case, the housingis connected to the air suction duct 32.

On the inner side of the drum 10 longitudinally extending vanes 45 maybe arranged along at least certain drum sections as shown in FIG. 5.These vanes which may, for example, have an angular or J-shapedcross-section as shown, but which may have any other suitable form,serve to a certain extent to carry along some of the material beingtreated in the drum during the rotation of the drum 10. As indicated inFIG. 5, the material carried by the vanes 45 will fall down from thevanes when they have been moved to an upper position, and the fallingmaterial will thus cover a smaller or larger part of the inner drumcross-section. The vanes 45 may extend along a helical or rectilinearpath, and the vanes may possibly be longitudinally offset in relation toeach other. To obtain good exploitation of the heat being produced bythe burner 17, the burner is preferably adjusted during the operation ofthe plant in such a manner that it is constantly directed approximatelytowards the centre 46 of the part of the drum cross-section which iscovered by the falling material.

Because the production plant described above and shown in the drawingsshould not only be used for drum-mixing, but also for batch-mixing, thedrum 10 should be able to treat two or more axially separated materialportions simultaneously in respective drum sections. To ensure goodseparation of the individual material portions, an annular partitionwall 47 like the one shown in FIG. 6 may be arranged on the inner wallof the drum between adjacent drum sections. This partition wall isdivided into a plurality of ring sectors 48, and each of these ringsectors is rotatable about a radially directed shaft or pivot pin 49.The ends of these shafts projecting from the outer side surface of thedrum, may be interconnected, for example by means of gears, chains orother connecting means, so that they may all be moved by rotating one ofthe shafts from a rotational position shown in FIG. 6 in which thepartition wall 47 is closed and all of the ring sectors 48 extend atsubstantially right angles to the drum axis, and into another rotationalposition in which the partition wall is open and all of the ring sectorsextend substantially parallel to the drum axis or form an acute angletherewith. The ring sectors 48 may then be moved from their closedposition to their open position when a material portion is to betransferred from one drum section to another as will be explained indetail below. Similarly, as indicated in FIG. 4, an end wall 50 may bearranged at the outlet end of the drum 10, which end wall is preferablyannular and which may be moved by means of a hydraulic cylinder 51 orsimilar moving means between a closed position in which the end wall isadjacent to the outlet end of the drum 10, and an open position in whichthe outlet end of the drum is open.

As shown in FIG. 7, the material intake 24 indicated in FIGS. 1-4 is astationary annular material introduction chamber 52 into which materialis introduced from the silo 25 by means of the conveyor 26. Thismaterial may fall into the drum 10 from the chamber 52 through anannular row of mutually spaced openings 53 which are formed in the drumwall. An obliquely extending baffle plate or bucket 54 projecting intothe inner space of the drum is aligned with each of the openings 53 andis shaped so as to move the material in a direction indicated by anarrow 55 towards the outlet end of the drum 10. Because of the freespaces 56 between the individual baffle plates or buckets 54, andbecause each of the baffle plates or buckets 54 is provided with asloping surface 57, material being treated in the drum 10 may almostfreely pass in an axial direction past the baffle plates or buckets 54without any substantial amounts of material being retained by the baffleplates.

The production plant shown in the drawings and described above operatesin the following manner:

If a large amount of bituminous paving mixture or asphalt mixture of auniform composition is to be prepared, it is advantageous to use thedrum-mixing principle. This means that sand and stone materials from thesilos 18 are continuously supplied to the inlet end of the drum 10 inthe ratios corresponding to those in the asphalt recipe. The drum 10 isrotated at a suitable rotational speed by means of the driving motors11, and the direction of the burner 17 is adjusted in such a manner thatan optimum heating effect is obtained at the resulting material flowrate. Further, the air and fuel supply to the burner is controlled insuch a manner in relation to the material flow that the stone and sandmaterial flowing through the drum 19 is heated and dried to the desiredextent. Bitumen, filler, recycled material in the form of used, brokenasphalt pavement, additives, etc., are continuously passed into the drumthrough the material intake 24 and the supply tube or tubes 58 andthrough any other corresponding material inlets. The finished hotasphalt mixture leaves the drum 10 continuously through its outlet endand is passed to the finished product silo 28 via the drag chainconveyor 27. When drum-mixing is used, any internal partition walls 47in the drum may be constantly in their open position, and the end wall50 of the drum may be constantly in its open position so that thematerials passed into the drum 10 may fairly freely move along throughthe drum, and the finished asphalt mixture may likewise flow out freelyfrom the outlet end of the drum.

If the production plant is to be used for the production of smallamounts of asphalt mixture (for example 2000-4000 kg) according to thesame recipe, the plant is advantageously operated according to thebatch-mixing principle. Metered amounts of sand and stone material inthe ratio prescribed by the recipe is metered from the silos 18 and thenpassed into the drum 10 by means of the conveyors 21 and 23. Thismaterial portion is introduced into the first drum section, which isadjacent to the inlet end of the drum 10. The drum 10 is now rotated ata suitable rotational speed, and the direction of the burner 17 isadjusted in dependency of the pattern formed by the falling materialacross the drum cross-section (vide FIG. 5) so that an optimumexploitation of the heat energy developed by the burner is obtained. Theamounts of air and fuel supplied to the burner are adjusted in order toobtain a desired degree of drying of the sand and stone materialstreated. In case of drum-mixing, the drum 10 is preferably given aconstant small inclination downward in the direction of the outlet endduring the whole process. However, in case of batch-mixing, thehydraulic cylinders 16 are preferably adjusted so that the drum axis issubstantially horizontal. This causes all the amount of material beingtreated in the first drum section to remain therein. As explained above,the drum sections may be separated by means of an annular partition wall47 as the one shown in FIG. 6. During operation of the drum thispartition wall will be in its closed position, and the end wall 50 ofthe drum will also be closed. When the desired temperature or degree ofdryness of the material portion present in the first drum section havebeen obtained, the inlet end of the drum 10 is lifted by means of thecylinders 16 so that the drum will incline towards the outlet end. Atthe same time the annular partition wall 47 is opened. This causes theheated and dried material portion to slide from the first drum sectioninto the second drum section. The partition wall 47 is now closed again,and a new portion of sand and stone material of a new composition may beintroduced into the first drum section from the silos 18. The hydrauliccylinders 16 may now be operated so as to bring the drum 10 back intoits substantially horizontal position. While the drying process justdescribed above is repeated for the new material portion which has beenintroduced into the first drum section, metered material portions to bemixed with the dry and heated sand and stone material in the second drumsection may be passed into this second drum section from the silo 25 bymeans of the conveyor 26, via the material intake 24 and/or through thesupply tube 58. These further materials may be one or more of thefollowing materials: Binders, such as bitumen, recycled materials in theform of broken, used asphalt pavement, filler, additives, etc. When thematerial portion in the first drum section has obtained the desiredtemperature and the desired degree of dryness, and when the materials inthe second drum section have been suitably mixed, the inlet end of thedrum is again lifted by means of the cylinder 16, and the end wall 50 ofthe drum 10 is opened so that the finished mixed material portion may beeither poured into the supply end of the conveyor 27 and be passed intothe product silo 28 or -- if the mixture is to have further substancesadded to it -- poured into the supply end of the conveyor 29 and bepassed into the buffer silo 41. From the silo 41 the mixture is passedinto the mixing apparatus 30 in metered amounts together with meteredamounts of the further desired additives, such as colouring agents,certain chemicals, etc. When the mixture has been poured out of thesecond drum section of the drum 10, the end wall 50 of the drum is againclosed, and the annular partition wall 47, if any, is opened so that theheated and dried material portion from the first drum section may slideinto the second drum section. After closure of the partition wall 47, anew portion of raw materials may be passed into the first drum sectionfrom the silos 18, whereupon the inlet end of the drum may again belowered, and the process described above may be repeated. To ensure thata good mixture of the material components is obtained in the mixingapparatus 30 without the mixing time being made longer than necessary,measuring probes may be arranged at different locations in the mixingapparatus, which probes measure the percentage content of one of thematerial components added. When the various measuring probes ascertainthe presence of substantially the same relative amount of the materialcomponent in question, the mixing process may be interrupted, and themixed material may be emptied into the inlet 42 of the conveyor 43 whichmay pass the finished portion of asphalt mixture into a product silo forfinished product, not shown.

It is understood that the drum 10 may be divided into more than two drumsections so that more than two material portions of differentcompositions may be treated simultaneously in the drum. Differentmaterial components may then be added and mixed in for example thesecond and the third drum sections. Alternatively, the plant may beoperated in such a manner in batch-mixing that only one material portionis present in the drum 10 at a time, this material portion, afterheating and drying in the first drum section, being poured directly intothe inlet end of the conveyor 29 and passed from there via the buffersilo 41 to the mixing apparatus 30. Both bitumen and filler and anyadditives may then be mixed together with the heated and dried sand andstone materials in the mixing apparatus 30.

The electronic control device or microcomputer 39 indicated in FIG. 3may control and monitor the operation of the whole plant. Thus a verylarge number of recipes for different types of paving or asphaltmixtures may be stored in the memory of the microcomputer. Based onthese recipes the microcomputer may control the metering of sand andstone material from the silos 18 which is passed to the inlet end of thedrum 10, the metering of bitumen, filler, pigment, latex and/or otheradditives via the intake 24 and/or the supply tube 58. Because the flowrates of the various materials through the system are different, thecomputer may also control the timing of releasing and supplying thevarious materials to be mixed so that they arrive at their finaldestination, i.e. the final mixing zone in the system at substantiallythe same time. This means that the desired composition of the mixtureproduced is obtained even for the first and last part of a large batchor amount of bituminous mixture being produced. Furthermore, thecomputer may control one or more of the following operations: Theaddition of further additives to the mixing apparatus 30, rotation andtilting of the drum 10, the function of the filter 33, recycling ofmaterial filtered off to the drum 10 or the mixing apparatus 30,adjustment of the burner 17, adjustment of supply of air and fuel to theburner 17, supply of surplus combustion air to the burner 17 based onmeasurement of CO or O₂ present in the combustion gas, the temperatureof combustion gas within the drum 10, opening and closing operations ofthe partition wall 47 and the end closure or end wall 50. The computer39 may further be adjusted to monitor a correct function of the variousunits and to give the alarm in the case of a malfunction.

The control device or the microcomputer 39 may further control thelocation of the supply tube(s) 58 and the time for the supply of meteredamounts of bitumen, filler, flux agents and other additives in the caseof batch-mixing. This control of the location and time may, for example,be made by means of equipment comprising one or more of the followingdevices:

(a) a microphone arranged at or inside the supply tube or tubes toregister noise from stone materials falling down on the outer surface ofthe tube.

(b) strain gauges arranged on the supply pipe or pipes for measuringbending stresses when the pipe or pipes are hit by stone materials.

(c) a rebound plate associated with an electrically or pneumaticallyoperating pulse system, by means of which pulses are counted or acontinuous signal is given when the rebound plate is hit by stonematerials.

It should be understood that various amendments and modifications of theembodiment described above and shown in the drawings could be madewithin the scope of the present invention as defined in the followingclaims.

I claim:
 1. A drying and mixing plant for the production of a bituminouspaving mixture, said plant comprising a drying and mixing drum (10)having inlet and outlet ends, driving means (11, 12) for rotating thedrum about its longitudinal axis, means (21-23) for feeding aggregatematerial into the drum at its inlet end, and means (24-26) for supplyinga further component material, such as a binder, into the drum atlocations intermediate of the inlet and outlet ends, characterized byinclination adjustment means (16) for lifting and lowering the drum (10)so as to change the inclination of the drum axis, closure means (50,FIG. 3) for selectively at least partly closing and opening the outletend of the drum (10), said drum (10) being divided into sections by atleast one annular partition wall (47) extending radially inward from theinner wall of the drum, said partition wall being selectively movablebetween a position in which flow of material between adjacent drumsections is obstructed and a position in which material may flow freelyfrom one section to another.
 2. A plant according to claim 1, furtherincluding heating means comprising at least one burner (17) directedinto the drum, characterized in that the burner (17) is mounted in sucha manner that its direction in relation to the drum axis may be altered.3. A plant according to claim 1, characterized in that the annularpartition wall (47) is divided into ring sectors (48) of which at leastsome are movable between an obstructing position in which they extendtransversely to the drum axis, and a non-obstructing position in whichthey extend substantially parallelly with or define an acute angle withthe drum axis.
 4. A plant according to any of the claims 1,characterized in that the means for supplying a further componentcomprises a peripherally extending baffle wall (54) projecting radiallyinward from the inner wall of the drum (10) and defining an introductionchamber opening towards the outlet end of the drum, the baffle wallbeing divided into sectors so as to define peripheral gaps (56) betweenadjacent sectors.
 5. A plant according to claim 1, characterized infurther comprising a mixing apparatus (30) communicating with the outletend of the drum by means of a conveyor (29).
 6. A plant according to anyof the claims 1, characterized in comprising an electronic controldevice (39) being adapted for comparing sensed or measured valuesagainst values, such as recipes, stored in the electronic control'smemory and in dependence of the comparison controlling and monitoringthe operation of the whole plant.
 7. A plant according to any of theclaims 1, characterized in that at the outlet end of the drum (10) anend wall (50, FIG. 4) or a closure may be arranged which may be moved inrelation to the drum between a closed position in which the outlet endof the drum is wholly or partially closed, and an open position in whichthe outlet end is open.